Thermoformed food trays having improved toughness

ABSTRACT

A continuous process for making a thermoplastic article comprises extruding a sheet of thermoplastic and contacting the sheet with a mold surface while the sheet is in a substantially non-oriented state. The mold surface is selectively heated and/or cooled during thermoforming to maintain the sheet in a molten or thermoformable state. A stripper plate adjacent to the mold surface is maintained at different temperature for inducing a predetermined degree of crystallinity to the sheet, for increasing web stiffness and improving web alignment, and optionally for assisting in the separation of the articles from the mold. A continuous apparatus for making thermoformed articles has co-extruders for extruding at least two distinct layers, which can have dissimilar properties (e.g., polar and non-polar), and a temperature controlled molding surface. Preferred thermoplastic compositions of the present invention have improved toughness and retained intrinsic viscosity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 09/722,594, filedNov. 28, 2000, which is a continuation-in-part of application Ser. No.09/535,953, filed Mar. 27, 2000, now U.S. Pat. No. 6,394,783, and acontinuation-in-part of application Ser. No. 09/453,457, filed Dec. 2,1999, now U.S. Pat. No. 6,576,309, the disclosures of each of which arehereby incorporated by reference.

FIELD OF THE INVENTION

The present invention is directed to thermoforming and, moreparticularly, to a continuous process and apparatus for thermoformingpolyesters into articles such as ovenable containers, food packagingtrays, and the like.

DESCRIPTION OF RELATED ART

Continuous vacuum-forming devices for making containers fromthermoplastic sheets typically utilize a continuous sheet of moltenplastic which is extruded and vacuum-formed on a continuous belt or arotating drum having a plurality of mold cavities. Many of these devicesutilize residual heat from the extrusion process, thus avoiding the needto reheat the plastic sheet prior to thermoforming. It generally isconsidered desirable that heat-set articles such as ovenable containershave relatively uniform thermal crystallinity throughout the article toprovide adequate dimensional stability and impact resistance. Forexample, Demerest U.S. Pat. No. 5,614,228 describes a continuous rotarythermoforming apparatus in which a sheet of molten polyethyleneterephthalate is extruded and vacuum-formed on a rotating drum having aplurality of mold cavities around its circumference. A hot oil circuitand electric heating elements are provided to impart different amountsof heat to different locations of the sheet during thermoforming.According to Demerest, additional heat is applied to portions of thearticles that have a greater wall thickness to produce more uniformcrystallinity throughout the article. The sheet is required to betensioned and oriented during article forming to prevent the sheet fromwarping or otherwise distorting during cooling. Orienting the sheet alsois said to result in articles having high impact resistance.

Several drawbacks exist with the type of thermoforming device describedby Demerest. For example, a minimum amount of crystallinity, which isstated to be at least about 20%, must be obtained in the article topermit the article to be removed from the mold cavity withoutsignificant distortion. Thus, the device is not useful for applicationswhere lower degrees of crystallinity may be desired in an article or aportion thereof. Moreover, the degrees of crystallinity actuallyobtained by using the Demerest apparatus typically are significantlyhigher than the stated minimum degree, and cannot be controlledeffectively. Another drawback is that forming the sheet under tensionresults in distortion of the article after molding, which limits theability of the apparatus to be used for many applications requiringespecially high tolerances.

The device described in Demerest also is limited in terms of productionspeed. Following thermoforming, the articles undergo a series of coolingand drying steps prior to being separated from the mold cavities. Theformed sheets are (again) tensioned to prevent distortion duringseparation from the mold. This type of procedure places severelimitations on production rates, especially for larger sized articles.

Dalgewicz U.S. Pat. No. 6,077,904 discloses a thermoforming process forpreparing polyesters that are said to have improved impact properties,low oxygen permeability, and low dimensional shrinkage during heating.According to Dalgewicz '904, impact modifiers are dissolved into moltenpolyester to form a eutectic alloy. On slow cooling, the eutectic alloyis said to freeze to form a mixture of particles of the impact modifierembedded in a matrix of the polyester. By controlling the solidificationof the melt, it is said that the size and distribution of precipitatesof impact modifier from the melt can be controlled to permit control ofthe mechanical properties of the composition.

Dalgewicz '904 suffers from several drawbacks. For one, the thermoformedpolyesters are extremely brittle, limiting their usefulness in manyapplications. The eutectic alloy formed requires the use of polyestershaving a high initial intrinsic viscosity (I.V.), and also makes thepolyesters more susceptible to thermal gradients upon the slow cooling.In addition, large 3D spheroids are developed in the polyesters,resulting in a high 3D morphology, which is undesirable in manyapplications. Yet another disadvantage of Dalgewicz '904 is that therequired cooling rate is very slow. Slow cooling increases the overalltime required for processing, which reduces efficiency and costeffectiveness.

Manlove U.S. Pat. No. 6,086,800 teaches a process and apparatus forcontinuously thermoforming articles. The apparatus has a plurality ofmold facets, each of which has (i) a static upper mold facet section and(ii) a dynamic lower mold facet section to which a mold cavity isattached. The two-part mold facet defines a relatively deep mold, i.e.,adapted to form deep-drawn articles. A thermoplastic sheet covers eachmold facet and is held in place by a vacuum groove located on the uppermold facet section. The material is shaped by actuating an assist plugin combination with a controlled evacuation of air from the mold cavity.

Manlove also suffers from numerous drawbacks. For example, the staticupper mold facet section is situated above the mold cavity. This meansthat the material is formed over the static upper mold facet and theninto the lower mold cavity, resulting in poor mold definition. Also, thestatic upper and dynamic lower mold facet configuration substantiallylimits production speed and increases waste, i.e., results in largeramounts of unused “trim” that must be discarded or recycled. Further,the lack of proximity of the upper mold facet section relative to thelower mold facet section prevents the upper mold facet section frombeing an effective means for influencing the temperature of thethermoplastic material within the mold cavity, in particular the portionthat is formed into the article. The mold facet configuration alsoencounters alignment difficulties at high temperatures due to thermalexpansion, which effectively limits the device to low temperatureapplications.

Gartland U.S. Pat. No. 4,469,270 describes a discontinuous thermoformingapparatus having a mold for thermoforming a plastic article having aflange portion. Vacuum and/or pressurized gas is used to conform a sheetto the shape of the heated mold. External cooling means are provided tomaintain a portion of the flange of the article at a temperature that issaid to be insufficient to induce undesirable thermal crystallization.This portion of the article preferably has a degree of crystallinity ofnot more than 10% to improve adhesion of lidding films to the article.The remaining portions of the flange and the remainder of the articleare said to preferably have the same average crystallinity.

It would be desirable to develop a continuous process and apparatus forthermoforming articles having excellent heat resistance and dimensionalstability. It also would be desirable to develop a continuous processand apparatus for thermoforming articles that exhibit excellent stressrelaxation and that do not undergo appreciable distortion duringcooling. It would be especially desirable to develop a process andapparatus capable of faster production times while substantiallyavoiding distortion, even for the production of larger sized articles.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention, a continuousprocess for preparing a thermoformed article comprises extruding athermoplastic layer through an extrusion die to form an extrudate in asubstantially non-oriented state. The extrudate is contacted with a moldsurface, such as a mold cavity (female mold) or a male mold. A stripperplate is disposed adjacent to the mold surface for controlling thetemperature of proximate areas of the extrudate, e.g., the area that isformed into the flange portion of a container. The stripper plateoptionally is also used for assisting in separating the articles fromthe mold surface by lowering the entire mold relative to the stripperplate. The extrudate remains in contact with the mold surface for a timesufficient to form the article. Ovenable containers and other articlesrequiring high temperature resistance typically are heat-set.

The temperature of the mold surface is controlled to maintain theextrudate in a thermoformable state. The mold surface temperature ortemperature gradient is controllably selected to induce a predetermineddegree of crystallinity or a predetermined crystallinity gradient in thearticle. The temperature of the mold surface thus is dependent on thephysical and chemical properties of the thermoplastic material(s) used,as well as the desired properties of the final article. The stripperplate is maintained at a different (usually lower) temperature than themold surface to control thermally induced crystallinity in proximateportions of the article. The temperatures of the mold and the stripperplate are suitably selected to achieve stress relaxation in the article,which permits the article to be separated from the mold without orsubstantially without distortion, independent of the level of thermallyinduced crystallinity.

In an alternative embodiment of the present invention, a multi-layeredarticle is formed in a continuous process by co-extruding at least twodistinct thermoplastic materials, followed by thermoforming under theconditions as described above. In one preferred embodiment, theco-extrudate comprises a polar thermoplastic layer, an intermediate tielayer, and a non-polar thermoplastic layer. The polar layer (e.g., PET)can form the external surface of the article, while the non-polar layer(e.g., polyethylene) can form the internal surface of the article, forexample to provide improved sealing properties with various liddingmaterials, especially using gas flushed sealing or modified atmospherepackaging (MAP). The present invention also is directed to a continuousapparatus for thermoforming multi-layered articles.

According to another aspect of the present invention, a thermoplasticpolymeric composition comprises an alkylene terephthalate or naphthalatebulk polymer, an additive, and a compatibilizer/emulsifier/surfactant(CES). The additive comprises a substantially amorphous co-polymer ofethylene and an acrylate. The CES comprises a grafted or backboneco-polymer or ter-polymer of ethylene and a glycidyl acrylate, maleicanhydride, or mixture thereof, and optionally an acrylate selected fromthe group consisting of methacrylate, ethylacrylate, propylacrylate,butylacrylate, ethylhexylacrylate, and mixtures thereof. When thecomposition is heat set and formed into a layer having a thickness ofabout 10 to 15 mils, the article preferably has a Gardner toughness(failure energy) at 73° F. (22° C.) of at least 110 in.-lb_(f), and at−20° F. (−29° C.) of at least 100 in.-lb_(f). Surprisingly, thecompositions exhibit improved toughness both at room temperature as wellas at low temperatures.

According to another aspect of the invention, a polyester-basedthermoplastic composition has a high retained viscosity following heatsetting. In particular, the heat set composition preferably has a finalintrinsic viscosity that is at least about 70% of the initial intrinsicviscosity of the bulk polymer. The high level of retained viscositypermits the conversion of polyesters having lower initial intrinsicviscosity, including resins that heretofore were unusable in food gradeand other applications.

The continuous thermoforming process of the present invention permitsthe sheet to be formed without the need for tensioning or orienting,resulting in significantly improved product definition and, especially,retained product definition. It is particularly preferred that the sheetnot be tensioned or oriented in either direction so as to essentiallyeliminate post-mold distortion. The process also improves accuracy andprecision of product trimming by reducing distortion normally attendantwith changes in thermally induced crystallinity as the sheet is heatedand cooled during processing. As a result, the articles can be separatedfrom the mold more rapidly following forming, which facilitates fasteroverall production rates.

The process of the present invention also permits more precise controlof thermally induced crystallinity in the articles. The degree ofcrystallinity in products can be tailored to a particular application,e.g., in the manufacture of food containers such as microwave-ovenablecontainers, dual-ovenable containers, and the like. Significantly, thedegree of crystallinity in the thermoformed article is not governed bymanufacturing limitations. For example, unlike conventional continuousthermoforming devices, no minimum degree of crystallinity is required toenable the articles to be separated from the mold without distortion.Rather, the degree of crystallinity in an article can be selectivelycontrolled in accordance with a degree most suitable for a particularapplication.

The present invention overcomes many of the limitations associated withthe prior art. For example, the present invention permits the conversionof lower melt strength materials, as well as the conversion of a varietyof thermoplastic materials at a much higher rate. Polyesters havingrelatively low crystallinity rates and polyesters having relatively highcrystallization rates can be used separately or in combination under theconditions described herein. In addition, the present invention permitsthe conversion of polyesters having lower initial intrinsic viscosity,including grades of resins that previously could not be used in foodgrade and other applications requiring high tolerances. Further, thepresent invention permits dissimilar materials (e.g., polar andnon-polar) to be processed in co-extruded form.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail withreference to preferred embodiments of the invention, given only by wayof example, and illustrated in the accompanying drawings in which:

FIG. 1 is an illustration of a continuous melt phase thermoformer inaccordance with one embodiment of the invention;

FIG. 2 is an illustration of a continuous melt phase thermoformer inaccordance with an alternative embodiment of the invention;

FIG. 3 is an illustration of a flat containing a plurality of moldingcavities in accordance with a preferred embodiment of the invention;

FIG. 4 is a side view of a mold member;

FIG. 5 is a side view of a mold member having electric heating elementsin accordance with an alternative embodiment of the invention;

FIG. 6 is an illustration of a mold member having a stripper plate inthe mechanical ejection position; and

FIG. 7 is a table showing the improved toughness properties of certainpreferred thermoplastic compositions of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A wide variety of thermoplastic materials can be used in the continuousthermoforming process of the present invention. Preferred thermoplasticmaterials include, by way of example, polyesters such as polyethyleneterephthalate (PET). The thermoplastic materials may include, in wholeor in part, virgin polymers, reprocessed or recycled polymers,post-consumer waste, and combinations thereof. As used herein,“reprocessed” and “recycled” each refer to materials that are unused ina given process cycle, typically the “trim” portions of the web aroundthe article-forming portions. The reprocessed materials typically arecollected, re-ground, and then mixed with virgin materials.

In many applications, it is desirable to use reprocessed materials notonly for cost savings, but also to provide more favorable kinetics toselectively control the level of thermally induced crystallinity. Forexample, virgin and reprocessed polyesters, such as PET, can be combinedat a ratio of virgin polyester to reprocessed polyester of from about1:4 to about 4:1 by weight, more typically from about 1:2 to about 2:1.In one preferred embodiment, virgin PET and reprocessed PET are combinedat a weight ratio of about 1:1.

The thermoplastic polymer(s) (“bulk polymer(s)”) can be homopolymers,co-polymers, or blends thereof, and may be straight-chained, branched,or mixtures thereof. In addition, blends of polymers having varyingmolecular weights and/or intrinsic viscosity (I.V.) may be used. WhenPET is used, I.V. most often ranges from about 0.5 to 1.2. The polymersmay be branched by inclusion of small quantities of trihydric ortetrahydric alcohols, or tribasic or tetrabasic carboxylic acids,examples of which include trimellitic acid, trimethylol-ethane,trimethylol-propane, trimesic acid, pentaerythritol and mixturesthereof. The degree of branching preferably is no more than about 3%. Ithas been found that blends of homopolymers and co-polymers areparticularly desirable to provide overall kinetics more favorable forcontrolling thermally induced crystallinity.

In one preferred embodiment, a thermoplastic composition comprises abulk polymer, an additive, and a compatibilizer/emulsifier/surfactant(CES). The thermoplastic composition is especially useful in thepreparation of articles having high dimensional stability and hightemperature resistance, which are particularly desirable attributes infood-grade applications (e.g., conventional-, convection-, microwave-,and dual-ovenable containers).

Unless otherwise indicated, all percentages set forth herein are weightpercentages based on the total weight of the thermoplastic composition.

As used herein alone or as part of another group, the term “alkyl” or“alk” denotes straight and branched chain saturated hydrocarbon groups,preferably having 1 to 20 carbon atoms, more usually 1 to 6 carbonatoms. Exemplary groups include methyl, ethyl, propyl, isopropyl,n-butyl, t-butyl, isobutyl, pentyl, hexyl, isohexyl, heptyl,4,4-dimethylpentyl, octyl, 2,2,4-trimethylpentyl, nonyl, decyl, undecyl,dodecyl, combinations thereof and the like.

The term “cycloalkyl” as used herein alone or as part of another group,denotes saturated cyclic hydrocarbon ring systems, preferably containing1 to 3 rings and 3 to 7 carbons per ring. Exemplary groups includecyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl,cyclooctyl, cyclodecyl, cyclododecyl, adamantyl and combinationsthereof.

The term “alkylene” as used herein denotes divalent, unsaturatedhydrocarbon groups of the overall formula —C_(n)H_(2n)—, wherein npreferably is from 1 to 10. Exemplary groups include methylene,ethylene, and propylene. Such groups represent alkyl groups as definedabove from which another hydrogen has been removed.

Intrinsic viscosity (I.V.) as used herein is defined as the limit of thefraction In (v)/C as C, the concentration of the polymer solution,approaches 0, wherein v is the relative viscosity which is measured atseveral different concentrations in a 60/40 mixed solvent of phenol andtetrachloroethane at 30° C. Units for I.V. are dl/g unless otherwiseindicated. “Initial intrinsic viscosity” and similar terms are used torefer to the I.V. of a polymeric material before processing (e.g.,before thermoforming or heat setting). “Final intrinsic viscosity” andsimilar terms are used to refer to the I.V. of a polymeric materialsubsequent to thermoforming. Unless otherwise clear from its context,intrinsic viscosity (I.V.) refers to the entire composition, e.g.,virgin materials plus any recycled or reprocessed materials, etc.

The bulk polymer can include, in whole or in part, an alkyleneterephthalate or naphthalate polyester. Polyalkylene terephthalates canbe prepared by the polycondensation reaction of terephthalic acid, or alower alkyl ester thereof, and aliphatic or cycloaliphatic C₂-C₁₀ diols.Such reaction products include polyalkylene terephthalate resins,including, but not limited to, polyethylene terephthalate, polybutyleneterephthalate, polytetramethylene terephthalate, and copolymers andmixtures thereof. As is known to those skilled in the art, thesepolyester resins may be obtained through the polycondensation reactionof terephthalic acid, or a lower alkyl ester thereof, and an alkylenediol. For example, polyethylene terephthalate can be prepared bypolycondensation of dimethyl terephthalate and ethylene glycol followingan ester interchange reaction. Non-limiting examples of suitablepolyesters include polyethylene terephthalate (PET), polybutyleneterephthalate (PBT), polyethylene naphthalate (PEN),polycycloterephthalate (PCT), polycycloterephthatlic acid (PCTA),(poly)ethylene-co-1,4-cyclohexanedimethylene terephthalate (PETG),polytrimethylene terephthalate (PTT), and co-polymers and mixturesthereof.

The bulk polymer may contain up to about 25 mol % of other aliphaticdicarboxylic acid groups having from about 4 to about 12 carbon atoms aswell as aromatic or cycloaliphatic dicarboxylic acid groups having fromabout 8 to about 14 carbon atoms. Non-limiting examples of thesemonomers include iso-phthalic acid (IPA), phthalic acid, succinic acid,adipic acid, sebacic acid, azelaic acid, cyclohexane diacetic acid,naphthalene-2,6-dicarboxylic acid, 4,4-diphenylene-dicarboxylic acid andmixtures thereof.

The bulk polymer also may contain up to about 25 mol % of otheraliphatic C₂-C₁₀ or cycloaliphatic C₆-C₂₁, diol components. Non-limitingexamples include neopentyl glycol, pentane-1,5-diol,cyclohexane-1,6-diol, cyclohexane-1,4-dimethanol, 3-methylpentane-2,4-diol, 2-methyl pentane-2,4-diol, propane-1,3-diol, 2-ethylpropane-1,2-diol, 2,2,4-trimethyl pentane-1,3-diol, 2,2,4-trimethylpentane-1,6-diol, 2,2-dimethyl propane-1,3-diol, 2-ethylhexane-1,3-diol, hexane-2,5-diol, 1,4-di(μ-hydroxy-ethoxy)benzene,2,2-bis-(4-hydroxypropoxy-phenyl)propane, and mixtures thereof.

Linear alkylene terephthalate or naphthalate homopolymers typicallyexhibit faster crystallization than do co-polymers. Branched polymerstypically yield higher melt strengths. As will be appreciated by thoseskilled in the art, mixtures of branched- or unbranched homopolymersand/or co-polymers, optionally having varying molecular weights and/orI.V., can be selected to obtain a polymer having the most suitableproperties for a particular application.

Polymers having lower I.V. generally have lower molecular weights,shorter chain lengths, and exhibit faster crystallization kinetics,resulting in better heat setting properties (e.g., higher dimensionalstability). In addition, lower-I.V. polymers generally are lessexpensive, and have lower extrusion melt temperatures, resulting in lessdegradation, faster stress relaxation time, reduced molding time andreduced production time. However, such polymers traditionally areconsidered unsuitable for making ovenable containers and other articlesrequiring high tolerances because of poor toughness, low melt strength,poor web handling characteristics, and poor ductility. The presentinvention advantageously overcomes the drawbacks conventionallyassociated with the use of lower-I.V. polymers, thereby permitting theiruse in such applications as food-grade, heat-set products.

The bulk polymer of the present invention may contain variousimpurities. Preferably, impurities that hinder crystallization are heldto a minimum. Examples of such impurities include acetylaldehyde,diethylene glycol, and isopropyl aldehyde, with preferred maximumconcentrations of these components being 2 wt %, 2 ppm, and 5 wt %,respectively, based on the total weight of the bulk polymer. Skilledpractitioners can easily identify the impurities that hindercrystallization and the concentration at which they do so. Otheradditives known in the art may be included in the composition up toabout 30% by weight. Non-limiting examples of such additives includeantioxidants, flame retardants, reinforcing agents such as glass fiber,asbestos fiber and flake, mineral fillers, stabilizers, nucleatingagents, ultraviolet light stabilizers, heat and light stabilizers,lubricants, dyes, pigments, toners, mold release agents, fillers, suchas glass beads and talc, and the like. Minor amounts of one or moreadditional polymers (e.g., up to about 10 percent by weight) optionallycan be incorporated in the present composition, such as polyamides,polycarbonates, polyethylenes, and polypropylenes. Antioxidants, thermalstabilizers, fillers, pigments and flame retardant additives, when used,preferably do not exert any adverse effect on impact strength.

The additive preferably comprises a co-polymer of an ethylene monomerand a co-monomer that forms a polar moiety such as an acrylateco-monomer. The additive imparts toughness to the thermoplasticcomposition and makes the composition particularly resistant to thermaltreatments that traditionally result in toughness reduction. The polaror semi-polar nature of the additive also improves dispersion andmixing. Examples of suitable co-monomers include acrylates such asmethacrylate, butylacrylate, ethylacrylate, ethylhexyl methacrylate, andmixtures thereof. The concentration of the co-monomer should be between(a) a minimum which depends upon the identity of the acrylate and (b) anamount slightly less than the amount that makes the co-polymer amorphousor substantially amorphous. For example, when methacrylate is used, itsconcentration preferably is from about 20 wt % to about 35 wt %, basedon the total weight of the ethylene/methacrylate co-polymer. Typicalpreferred acrylate concentrations range from about 7 wt % to about 40 wt% and more typically from about 17 wt % to about 35 wt %, based on thetotal weight of the co-polymer. The average molecular weight of theco-polymer typically ranges from about 50,000 to about 120,000. The meltflow index of the additive preferably is less than about 7, morepreferably is less than about 3, and even more preferably is less thanabout 2 g/10 min. The additive preferably has a relatively low meltingpoint and is thermally stable, e.g., does not degrade during extrusionof the thermoplastic composition or during re-drying of thethermoplastic composition in air for extended times.

The concentration of the additive component in the thermoplasticcomposition may be suitably selected according to properties requiredfor desired end uses of the composition. Typically, the concentration ofthe additive is from about 4 wt % to about 40 wt %, more typically fromabout 4 wt % to about 30 wt %, and even more typically from about 6 wt %to about 15 wt %, based on the total weight of the composition.

The additive co-polymer most often has a major portion of ethylene,typically at least about 60 wt % and more usually at least about 65 wt%, based on the total weight of the additive. The co-polymer also maycontain one or more alpha-olefins having 3 to 10 or more carbon atoms.Illustrative examples include propylene, butene-1,pentene-1,3-methylbutene-1, hexene-1, octene-1,decene-1,4,4-dimethylpentene-1,4,4-diethyl-hexene-1,3,4-dimethylhexene-1,4-butyl-1-octene, 5-ethyl-1-decene,3,3-dimethyl-butene-1, mixtures thereof and the like. The preferredethylene co-polymer comprises up to about 5 wt % of other alpha-olefinsas described above. In addition, the additive optionally contains acore-shell toughener. Examples of core-shell tougheners that can be usedare described in U.S. Pat. No. 5,409,967, the disclosure of which isincorporated by reference herein in its entirety.

The compatibilizer/emulsifier/surfactant (CES) preferably is a graftedor backbone-based co-polymer or ter-polymer comprising ethylene and aglycidyl acrylate, such as glycidyl methacrylate, and/or maleicanhydride. The CES co-polymer or ter-polymer preferably also includesother acrylates such as methacrylate, ethylacrylate, propylacrylate,butylacrylate, ethylhexylacrylate, etc. Suitable exemplary amounts ofglycidyl acrylate, maleic anhydride, or mixture thereof, range fromabout 0.05 wt % to about 12 wt %, typically from about 0.5 wt % to about10 wt %, and more typically from about 0.8 wt % to about 9 wt %, basedon the total weight of the co-polymer or ter-polymer. A graftedco-polymer or ter-polymer typically will have less glycidyl acrylate ormaleic anhydride (e.g., from about 0.2 wt % to about 1.5 wt %), whereasa backbone-based co-polymer or ter-polymer can have higher amounts ofglycidyl acrylate and/or maleic anhydride, e.g., as indicated above.Suitable exemplary amounts of acrylate range from 0 to about 40 wt %,preferably from about 10 wt % to about 30 wt %, and even more preferablyfrom about 20 wt % to about 35 wt %, based on the total weight of theco-polymer or ter-polymer.

Typically, the concentration of the CES in the thermoplastic compositionis from about 0.1 wt % to about 8 wt %, more typically is from about 0.2wt % to about 6 wt %, and even more typically is from about 0.4 wt % toabout 4 wt %. The melt flow index of the CES preferably is less thanabout 20, more preferably is less than about 15, and even morepreferably is less than about 10 g/10 min.

In one preferred embodiment of the present invention, the CES comprisesa ter-polymer of ethylene with about 8 wt % glycidyl methacrylate andabout 25 wt % methacrylate or butylacrylate, based on the total weightof the ter-polymer. In another preferred embodiment, the CES comprises aco-polymer of ethylene with about 6 wt % glycidyl methacrylate, based onthe total weight of the co-polymer. In another preferred embodiment, theCES comprises a ter-polymer of ethylene with about 2 wt % glycidylmethacrylate and from about 17 to 25 wt % methacrylate, based on thetotal weight of the ter-polymer. In yet another preferred embodiment,the CES comprises a ter-polymer of ethylene with about 3 wt % maleicanhydride and about 17 wt % butylacrylate, based on the total weight ofthe ter-polymer.

In an alternative preferred embodiment, the CES (as previously defined)is melt blended with a polar co-polymer, for example a co-polymer ofethylene and a C₁-C₁₂ acrylate, such as ethylene methacrylate (EMA) orethylene butylacrylate (EBA). For example, the CES and polar co-polymercan be melt blended at a weight ratio of about 1:9 (CES to co-polymer).This embodiment improves uniformity of the CES and is particularlyuseful in co-extruding the CES as a separate tie layer in amulti-layered article, as discussed in greater detail below.

Preferred thermoplastic compositions of the present invention exhibitnot only improved toughness, but also improved low-temperaturetoughness. When the composition is heat set and formed into a layerhaving a thickness of about 10 to 15 mils, articles typically exhibit aGardner toughness (failure energy) at 73° F. (22° C.) of at least 110in.-lb_(f) and preferably at least about 115 in.-lb_(f); and alsotypically exhibit a Gardner toughness at −20° F. (−29° C.) of at least100 in.-lb_(f), preferably at least about 105 in.-lb_(f) (see FIG. 7).

For compositions heat-set at a thickness of about 15 to 25 mils, theDynatup Impact toughness rating at 70° F. (21° C.) preferably is atleast 125 and at −20° F. (−29° C.) preferably is at least 120.Preferably, the Dynatup Impact toughness rating at 70° F. (21° C.) is atleast 130 and at −20° F. (−29° C.) is at least 125. Surprisingly,compositions of the present invention were found to have Dynatup Impacttoughness rating at −20° F. (−29° C.) of at least 130, 140, 150, andeven higher.

Preferred thermoplastic compositions of the present invention alsoexhibit improved retained I.V. In particular, the heat set article has afinal intrinsic viscosity that is at least about 70%, preferably atleast about 75%, and even more preferably at least about 80% of theinitial intrinsic viscosity of the thermoplastic composition. Thedifference between initial I.V. and final I.V. preferably is less thanabout 0.15, more preferably less than about 0.12, even more preferablyless than about 0.1.

It is particularly surprising that the preferred compositions of thepresent invention exhibit the improved toughness, as discussed, while atthe same time exhibiting improved retained viscosity or final viscosity.The present invention also enables the conversion of lower I.V.polyesters in a continuous process into products exhibiting toughness,and especially low temperature toughness, that heretofore could not beobtained in a continuous process.

Heat setting is a term describing the process of thermally inducingcrystallization of a polyester article in a restrained position. In thepractice of the invention, heat-setting can be achieved by maintainingintimate contact of the solid or cellular sheet with the heated moldsurface for a sufficient time to achieve a level of crystallinity whichgives adequate physical properties to the finished part. For containersto be used in high temperature food applications, a level ofcrystallinity above 15% is preferable for adequate dimensional stabilityduring demolding operations, and more preferably is above about 20% toyield parts with excellent dimensional stability and impact resistance.

The heat-set part can be removed from the mold cavity by known means.One method, blow back, involves breaking the vacuum established betweenthe mold and the formed sheet by the introduction of compressed air. Ina typical commercial thermoforming operation, the part is subsequentlytrimmed and the scrap ground and recycled.

Since a partially-crystalline finished article is necessary for gooddimensional stability at high temperatures, knowledge of the degree ofcrystallinity or percent of crystallinity is of considerable importance.The crystallinity of the polymer in such articles will normally bemeasured by Differential Scanning Calorimetry (DSC). The termscrystallization temperature and crystallization onset are usedinterchangeably to mean the temperature or temperature range in which aregularly repeating morphology, brought about by a combination ofmolecular mobility and secondary bonding forces, is induced in a polymerover a molecular distance of at least several hundred angstroms. In PET,for example, the crystallization temperature or crystallization onsetcan be visually observed as the point at which a substantiallyamorphous, non-oriented sheet of polymer changes from a translucent,hazy appearance to a white appearance.

The term glass transition temperature (T_(g)) is used herein to refer tothat temperature or temperature range at which a change in slope appearsin the volume versus temperature curve for a polymer, and to define atemperature region below which the polymer exhibits a glassycharacteristic and above which the polymer exhibits a rubberycharacteristic. The glass transition temperature of polyethyleneterephthalate is about 70 to 80° C.

The temperature of the mold surface should be selected in accordancewith the properties of the bulk polymer(s) and the desired level ofcrystallinity in the thermoformed article or a region thereof. Thetemperature of the mold surface should be above the softening point andstretching point of the bulk polymer(s). Such temperatures promotestress relaxation in the thermoplastic material, which in turneliminates or substantially eliminates post-mold distortion. Preferably,the lowest suitable temperature for the mold surface or a region thereofis selected, so as to avoid the need for additional cooling.

The stripper plate should be maintained at a temperature that isdifferent than the temperature(s) of the mold surface. Preferably, thestripper plate is maintained at a temperature that differs from thetemperature of an adjacent portion of the mold surface by apredetermined amount. The difference in temperature can vary over a widerange, and depends on such factors as the level of crystallinity and theend level of heat resistance desired in the product. The difference intemperature most often ranges from about 1° C. to about 100° C., moreusually from about 5° C. to about 68° C.

When female molds (mold cavities) are used and/or when high temperatureresistance is desirable in the article, it often is preferable tomaintain the stripper plate at a temperature lower than the temperatureof the mold surface. One of the advantages of using female molds is thatin-mold shrinkage, associated with high levels of crystallinity, pullsthe thermoplastic material away from the mold rather than into the mold,the latter of which creates stresses in the article. Preferably, thestripper plate temperature is not lower than the glass transitiontemperature (T_(g)) of the thermoplastic material. Stripper platetemperatures below the glass transition temperature may requirere-drying and/or re-crystallization of the thermoplastic material, whichcan increase process time and cost. The cooler stripper plate not onlyselectively limits thermally induced crystallinity in adjacent (e.g.,flange) portions of the article, but also increases web stiffness,improves web alignment, and can improve article extraction (in apreferred embodiment discussed below).

When male molds are used and/or when amorphous or substantiallyamorphous products are desired (including many applications in whichhigh temperature resistance is not required), it often is preferable tomaintain the stripper plate at a higher temperature than the temperatureof the mold surface. Where high temperature resistance is not required,blends of materials (e.g., virgin and reprocessed; homopolymers andco-polymers) often can be converted at lower mold temperatures, whichtranslates to higher production speed and lower production cost. Iftransparency is desired, reprocessed materials usually are not used, andthe thermoplastic material can be further processed, as needed, inaccordance with techniques well known to those skilled in the art. Italso is contemplated that a male mold can be used as a plug assist.

In accordance with a preferred embodiment of the invention, the bulkpolymer comprises polyethylene terephthalate (PET) having an I.V. ofless than 0.95, 0.90, 0.85, or 0.80 (available from Shell Polyester).The components (e.g., bulk polymer(s), additive(s), CES, etc.) can beblended in either a batch or continuous manner: The order of addition ofthe components is not critical. Preferably, dry components are blendedsimultaneously or sequentially, followed by physically dry blending thebulk polymer by weight. The components then can be dried, melt mixed,devolitized, and processed through an extrusion die to form a sheet ofdesired thickness, using a single screw extruder, a twin screw extruder,or a multi-machine system (co-extruder). It may be advantageous to usemultiple extruders, either of the same type or of different types. Forexample, a twin-screw extruder can be used in combination with asingle-screw extruder to improve mixing of reprocessed or recycledpolymers, additives, and the like. In some instances it may be necessaryto use a twin-screw extruder in applications requiring especially highoutputs.

The residence time of the composition in the extruder(s) can vary over awide range, depending on such factors as the properties of thecomponents and the type of extruder(s) used, and is not critical to thepractice of the invention. The residence time preferably is kept to aminimum time that is sufficient to obtain proper mixing of thecomponents. Generally, residence times will be higher in single-screwextruders and lower in twin-screw extruders. Residence time sometimescan be as much as 6 minutes.

It also is preferred that shear be kept to a minimum. It is preferredthat the CES functions to break down particle size, rather than usingshear to break down particle size. Dispersive mixing optionally isperformed and, when performed, typically is done at the initial stage ofmixing. It is preferred that no more than about 10-20% of mixing bedispersive, with the balance being distributive mixing, based on thetotal mixing time. Dispersive mixing can be used, for example, to breakdown particle size of pigments, fillers, inorganic materials, and thelike. Dispersive mixing most often has a shear rate of from about 400 to500 sec⁻¹ and distributive mixing most often has a shear rate of fromabout 200 to 400 sec⁻¹.

Optionally, a multi-machine system can be used to apply a skin on one orboth sides to form a multi-ply article, e.g., to enhance sealing oraesthetics, or to provide gloss, color, and the like. For example,because higher-crystallinity films usually are more difficult to seal,it may be advantageous to apply a second, more amorphous film, such asPETG or an iso-phthalic acid (IPA)/PET co-polymer, over a morecrystalline first layer.

Articles formed by the continuous process of the present invention canhave a wide variety of shapes and dimensions, facilitating a widevariety of end uses. Shallow articles, such as trays, generally can beprocessed at higher speeds. As will be apparent to those skilled in theart, the manufacture of articles having a greater area stretch ratio(i.e., deeper drawn parts) often requires the use of a plug assist.Typically, the area stretch ratio ranges from about 1.25:1 to about 3:1,more usually from about 1.5:1 to about 3:1, in accordance with thepresent invention.

With reference to FIG. 1, in accordance with a preferred embodiment ofthe present invention, a plastic material to be thermoformed isprocessed through an extrusion die 10 to form a plastic sheet 8 ofdesired thickness. The extrusion die 10 receives molten thermoplasticmaterial from an extruder 5. The extrusion die forms a plastic sheet 8that exits the die 10 in a plane disposed at any suitable angle αrelative to the horizontal axis h. To provide thermoformed articleshaving excellent dimensional stability, especially at elevatedtemperatures, it is particularly preferred that the plastic sheet 8 notbe stretched or oriented in either direction. The temperature of thematerial exiting the die 10 depends on such factors as the melting pointof the resin(s), and typically ranges from about 450 to 530° F. (about232 to 276° C.).

Upon exiting the extrusion die 10, the plastic sheet 8 optionally is fedover one or more rolls 30 or pairs of rolls 20 rotatably supported onthe apparatus. The rolls 20, 30 can be used to shape and cool thesurface of the plastic sheet 8 to establish a thermal gradient therein.The plastic sheet 8, however, should remain at a temperature suitablefor vacuum forming, i.e., in a molten or thermoformable state. The rolls20, 30 also can be used to laminate additional plastic sheet(s), toemboss the article, and the like. As will be apparent to those skilledin the art, other types of devices additionally or alternatively can beused to treat the plastic sheet 8, e.g., subsequent to extrusion andprior to thermoforming, without departing from the spirit or scope ofthe invention.

With reference to FIGS. 1, 3, and 4, the plastic sheet 8 is brought intocontact with a rotating wheel 50 having a plurality of mold members 300each having a forming cavity 310 (female mold). As will be understood bythose skilled in the art, the forming cavity 310 is perforated or ventedso that vacuum may be drawn in the mold. A vacuum device (notillustrated) is provided for drawing a vacuum through perforations (notillustrated) in the forming cavity 310. The vacuum underpressure drawsthe plastic sheet 8 into the forming cavity 310 to form an article inthe shape of the forming cavity 310. In this way, the plastic sheet 8 isdrawn into contact with the mold while any air trapped between theplastic sheet 8 and the forming cavity 310 is removed through theperforations.

As illustrated in FIG. 3, in accordance with one embodiment of theinvention, a flat 200 contains five parallel, spaced mold members 210for receiving the extruded sheet 8. It will be apparent to those skilledin the art that fewer or more mold members 210 can be provided on a flat200 as desired. A flat 200 can contain as few as one mold member, andthere is no upper limit contemplated on the number of mold members 210per flat 200. Selection of a suitable number of mold members 210 perflat 200 can be made according to such factors as, for example, cost,throughput, size of the thermoformed articles, size of the apparatus,energy requirements, etc. The rotating wheel preferably is of a sizesuitable to contain a plurality (e.g., 10 to 30 or more) of flats 200arranged around its circumference. In one preferred embodiment, therotating wheel contains 28 flats each having five mold members 210.

In accordance with one embodiment of the invention, while the plasticsheet 8 is in contact with the forming cavity 310, regions of theforming cavity 310 are selectively heated so as to increase the rate ofthermal crystallization relative to other regions to achieve the desireddegree of crystallinity in each region. Regions of the forming cavity310 also can be selectively cooled to decrease the rate of thermalcrystallization in the region. As will be understood by the art, thedegree of crystallinity imparted to a particular region of the articleis a function of not only the thermoforming temperature, but also theidentity and properties of the thermoplastic material, e.g., intrinsicviscosity (I.V.) and the like, and its thickness in the region.

The time that the sheet remains in contact with the mold surface at themolding temperature can vary over a wide range and depends on a numberof factors, such as the molding temperature, the dimensions of the mold,the number of molds, and the like. In a preferred apparatus of thepresent invention, the time at the molding temperature typically rangesfrom about 10-30 seconds, more usually from about 10-20 seconds.

The temperature in the mold cavity 310 can be controlled by any suitableheat transfer elements, e.g., heating, cooling, and/or insulatingelements. For example, as illustrated in FIG. 4, a fluid such as oil canbe supplied to a manifold 355 by hoses 356, and delivered through tubes350 and into channels (not illustrated) extending through the mold 310.The configuration and location of the channels can be suitably selectedto maintain a desired temperature or temperature distribution in themold cavity 310.

FIG. 5 illustrates another embodiment in which a heat transfer fluid iscirculated through channels 346 extending through a heat transfer plate345, which can be bolted or otherwise attached to the mold 310. In thisembodiment, the mold cavity 310 is heated by conduction. The mold 310and/or heat transfer plate 345 also may be equipped with optionalelectric heating elements (not illustrated) to selectively heat portionsof the forming cavity 310. In any embodiment, fluid (e.g., oil) andelectric heating elements can be used separately or in combination, orany other suitable means can be used for selectively heating and/orcooling portions of the forming cavity 310 in accordance with thepresent invention.

As illustrated in FIG. 4, a predetermined temperature distribution inthe forming cavity 310 and stripper plate 320 can be obtained by usingan appropriate combination of heating, cooling, and insulating elements.Insulator blocks 330, 330 a are attached to and disposed between theheated mold and the mounting plate 340, and between the mounting plate340 and the hot oil manifold 355, respectively. In addition, a heattransfer medium, such as water, optionally is circulated through acircuit (not illustrated) provided in the mold mounting plate 340 toselectively cool the mounting plate 340 and the stripper plate 320. Asillustrated in FIGS. 4-6, the stripper plate 320 preferably is taperedto assist in establishing a desired temperature distribution therein.The top portion of the stripper plate 320 is the thickest, and thus thecoolest portion. A heat transfer pin (not illustrated) also may bedisposed in the mold 310, e.g., parallel to the stripper plate 320, toestablish a desired temperature distribution in the stripper plate 320.

A feedback control device, such as a programmable logic controller(PLC), optionally is provided in combination with the heating and/orcooling means for controlling the temperature in the various portions ofthe forming cavity 310. Such temperature control devices can increasethe cost of the apparatus, but also can enable even more precise controlof crystallinity in the various regions of the article, as well asimprove consistency among articles produced by the apparatus.

In one embodiment, a thermoformed container has three distinct regionsof thermally induced crystallinity. In this embodiment, the temperaturedistribution is such that the bottom portion of the forming cavity 310is the hottest during thermoforming, typically from about 250 to about450° F., while the sides and the upper portions of the forming cavity310 are maintained at lower temperatures, with optional cooling. Thebottom portion of the thermoformed article typically has greater thanabout 20% crystallinity, preferably from about 22% to about 35%. Thebottom portion of the article thus has the highest heat resistance. Thebottom portion of the article is substantially stress-free and maintainsshaped-part (dimensional) stability, especially at elevated temperaturessuch as those typically encountered in cooking applications.

In this embodiment, the upper portion of the forming cavity 310preferably is the coolest portion during thermoforming, resulting in atop (e.g., flange) region having lower thermal crystallinity.Crystallinity in the top region preferably is less than about 30% andmore preferably is less than about 25%. The intermediate region of thearticle preferably is maintained at a thermoforming temperature betweenthat of the bottom portion and that of the top portion of the formingcavity 310. The intermediate region of the thermoformed article thus hasa degree of crystallinity between that of the bottom region and that ofthe top region. Within the intermediate region, a substantially uniformdegree of crystallinity can be present, e.g., by maintaining asubstantially uniform temperature within the intermediate region duringthermoforming. Alternatively, the intermediate region can have acrystallinity gradient, e.g., which ranges from the degree ofcrystallinity in the bottom region to that in the top region. In manyapplications, however, it is undesirable to have an appreciable gradientin crystallinity over a region of the article. Of course, where anarticle has multiple regions of crystallinity, aminor gradient willexist between the regions. The gradient of crystallinity in thethickness direction of any given region of the article preferably isnegligible.

The present invention has been described primarily with reference tofemale molds (mold cavities). It also is contemplated that male moldscan be employed in an analogous manner. A reverse temperature profilecan be employed for male molds to form articles having similarcrystallinity or regions of crystallinity as described above for femalemolds. For some applications, it may be desirable to have an inversedcrystallinity gradient compared to that previously described, e.g., ahigher degree of crystallinity at the top portion of the article and alower degree of crystallinity at the bottom portion.

It also may be desirable to use a male mold configuration forapplications not requiring high heat resistance, so as to increaseproduction speed. An example of such an application is the conversion ofamorphous polyethylene terephthalate (APET) into transparent containers.In this embodiment, the stripper plate contacts the surface of the sheetforming the inside of the article, instead of the surface of the sheetforming the outside of the article, as typically is the case with thefemale mold configuration described above.

Other non-uniform distributions of crystallinity may be obtained so asto provide articles especially suited for particular purposes. It willbe apparent to persons skilled in the art that the temperature and/ortemperature gradient in various regions can be suitably adjusted byselective heating and/or cooling to obtain the desired degree ofcrystallization in each region. In one preferred embodiment, thepredetermined temperature distribution in the mold results inthermoformed articles having at least two, and more preferably at leastthree, distinct regions of thermal crystallinity. Depending on suchfactors as the shape and the intended use of the thermoformed article,it may be advantageous to thermoform articles having four, five, six oreven more distinct regions of thermal crystallinity.

In applications requiring high temperature resistance, the article ismaintained in the forming cavity 310 for a time sufficient to form andheat-set the article. The article 1 then is separated from the formingcavity 310, e.g., at an ejection station 70, by action of the stripperplate 320. FIG. 6 illustrates the mold member 300 with the stripper 320plate in the mechanical ejection position. The mold 310 is displaced(e.g., toward the axis of the rotating wheel 50) in relation to thestripper plate 320 by a distance t sufficient to separate the article 1therefrom. By controlling the temperature of the stripper plate and notimparting tension to the web, the article 1 can be separated from themold 310 without or substantially without distortion. Thetemperature-controlled stripper plate 320 also facilitates removal ofthe articles 1 in less time after forming, thereby increasing productionrates.

The article 1 can be removed from the forming cavity 310 by any suitablemeans, with or without in-mold trimming. In one embodiment, the articles1 and the web of plastic between them (the “trim”) are separated fromthe forming cavity 310 as a unit. The web can be fed through a trimpress guide 90 to trim press 100 to remove the trim from the articles 1.Alternatively, the articles 1 can be trimmed while still in the formingcavity 310 by a suitable in-mold trimming device (not illustrated). Ineither embodiment, the articles 1 can be treated in an optionalpost-mold conditioning/treatment unit 80 which may provide one or moreof heat treatment, heat removal, perforating, or the like.

Cellular Sheets

The process and apparatus of the present invention can be used toprocess cellular sheets. Cellular sheeting can be made, for example, bymixing at least one inert gas with a molten thermoplastic resincomposition in an extruder. This is done by simply injecting the inertgas into the molten resin in the extruder that is equipped with asheet-forming die. The inert gas used in this process can be any gasthat does not chemically react with the thermoplastic resin compositionat the elevated processing temperatures required. Some representativeexamples of inert gases that can be used include nitrogen, carbondioxide, helium, neon, argon, and krypton. For purposes of cost savingsand solubilities, nitrogen, carbon: dioxide, or mixtures thereofnormally will be used as the inert gas.

A cellular sheet can be made with either a plasticating extruder or amelt extruder. Screw extruders of these types push the moltenthermoplastic resin composition containing discrete cells of the inertgas through a metal die that continuously shapes the sheet into thedesired form. In most cases, single screw extruders will be utilized.However, in some cases it may be desirable to utilize twin screwextruders or multiple screw extruders that perform essentially the samefunction.

In many cases it will be convenient to employ a plasticating extruder ofthe single screw design. The thermoplastic resin composition is fed intosuch a plasticating extruder by gravitational flow from a hopper intothe screw channel. The thermoplastic resin composition fed into theplasticating extruder is initially in particulate solid form. Thethermoplastic resin composition initially enters the solid conveyingzone of the plasticating extruder. In the solid conveying zone, thesolid resin is conveyed and compressed by a drag-induced mechanism. Inthe solid conveying zone, the resin is mixed, heated, and conveyedthrough the extruder toward the melting zone. This heating is providedby maintaining the barrel of the extruder at an elevated temperature.The barrel of the extruder is typically heated electrically or by afluid heat exchanger system. Thermocouples are also normally placed inthe metal barrel wall to record and to help control barrel temperaturesettings.

Melting occurs in the melting zone after the resin is heated to atemperature above its melting point. In the melting zone, melting,pumping and mixing simultaneously occur. The molten resin is conveyedfrom the melting zone to the melt conveying zone. The inert gas isinjected into the molten resin in the melt conveying zone. In the meltconveying zone, pumping and mixing simultaneously occur. The moltenresin in the melt conveying zone is maintained at a temperature wellabove its melting point. A sufficient amount of agitation is provided soas to result in an essentially homogeneous dispersion of inert gasbubbles throughout the molten resin. The molten resin entering the meltconveying zone from the melting zone is at a somewhat lower temperatureand accordingly is of a higher viscosity. This essentially prevents theinert gas from back mixing through the extruder and escaping from thesolid conveying zone via the hopper.

The molten thermoplastic resin composition in the melt conveying zonetypically is pumped into a metering pump and finally extruded through asheet-forming die. The metering pump and sheeting die are typicallymaintained at a lower temperature than that of the barrel surroundingthe melt conveying zone to minimize rupture and diffusion of inert gasbubbles in the thermoplastic resin composition. The sheeting die is of agenerally rectangular design that is quite wide and of a small opening.Upon exiting the sheeting die, the sheet extrudate will swell to a levelthat is dependent upon the melt temperature, the die length-to-openingratio, and the shear stress at the die walls. In some cases, such as inthe manufacture of clam shells, it is desirable to use a circular dieand to extrude a tube that can be slit open and thermoformed. Thecellular sheet produced typically is cooled without stretching byconvected cold air or an inert gas, by immersion into a fluid bath, orby passage over chilled rolls. The cellular sheet produced is generallyamorphous in nature.

The cellular sheet typically will contain a sufficient amount of inertgas cells to provide it with a density within the range of about 0.1 toabout 1.25. In most cases, the cellular sheet will contain a quantity ofinert gas cells to provide it with a density within the range of 0.2 to1.1. It generally is preferred for the cellular sheet to have a densitywithin the range of about 0.3 to about 1.0.

The cellular sheet can be thermoformed into heat-set, thin walledarticles utilizing conventional thermoforming equipment. Suchthermoforming typically is done by (1) preheating the substantiallyamorphous cellular sheet until it softens and positioning it over themold; (2) drawing the preheated sheet onto the heated mold surface; (3)heat-setting the formed sheet by maintaining sheet contact against theheated mold for a sufficient time period to partially crystallize thesheet; and (4) removing the part out of the mold cavity. In currentlyavailable thermoforming processes, the level of crystallinity of thepreformed sheet should not exceed about 10%.

The preheating of the substantially amorphous, cellular sheet prior topositioning over the thermoforming mold is necessary in order to achievethe very short molding times required for a viable commercial process.The sheet must be heated above its T_(g) and below the point at which itsags excessively during positioning over the mold cavity. In thethermoforming process, a sheet temperature within the range of about130° C. to about 210° C. and a mold temperature within the range ofabout 140° C. to about 220° C. will normally be utilized. It is oftenpreferred to use a sheet temperature within the range of about 155° C.to about 185° C. and a mold temperature within the range of about 165°C. to about 195° C.

Multi-Layered Articles

Multi-ply articles can be produced in a continuous process byco-extrusion of two or more distinct layers. This technique may be used,e.g., for aesthetic purposes, such as in making a two-tone, ovenablecontainer. If desired, two polymeric layers can be co-extruded to“sandwich” a third layer. Optionally, a food-grade composition can beextruded over a non-food grade composition to prepare an ovenablecontainer. Preferred thermoplastic compositions of the present inventionhave good sealability, e.g., permit packaging of refrigerated foodsunder pressure and the like. In some instances it may be desirable toextrude a more amorphous layer over a highly crystalline layer, e.g., asin hermetic sealing. Such additional layers may be selected from a widevariety of oriented and non-oriented films of homo-polymers,co-polymers, and mixtures thereof which can be straight-chained,branched, or mixtures thereof. Examples of such polymers includepolyesters such as PET, PEN, PETG, PCT, PCTA, PBT, PTT, and mixturesthereof. Suitable methods that can be used for co-extrusion aredescribed in U.S. Pat. Nos. 4,533,510, 4,929,482, and 5,318,811. Amulti-ply article can have one or more solid layers and/or one or morecellular layers, which can be sequenced in any desired configuration.

With reference to FIG. 2, a co-extruder 5′ having a single screw, a twinscrew, or a combination of both, is used for co-extruding amulti-layered plastic sheet 8′. The layers of the sheet 8′ can be solidlayers, cellular layers, or any combination thereof. The sheet 8′ can befed between a pair of shaping and cooling rolls 20, followed by a roll30′ which can further cool, shape, and/or emboss the sheet 8′, and whichoptionally laminates one or more additional thermoplastic sheets (notillustrated) to the co-extruded sheet 8′. The apparatus also can includeone or more conditioning stations 60 disposed around the rotating wheel.By way of example, the conditioning stations 60 can apply coatings,in-mold labels, paperboard, foil inserts, and the like. An adjustablemechanical ejection station 70′ ejects the articles 1 from the formingapparatus 50. The ejection station 70′ is mounted such that its positionalong the circumference of the rotating wheel can be adjusted, e.g., topermit rearrangement of the apparatus, maintenance, and the like. Uponexiting the forming apparatus 50, the formed articles 1 optionally arefed into a unit 80′ having one or more post-mold treatments such asperforating, heating, heat removal, or the like. As in the previousembodiments, the article should not be tensioned at any time to avoiddistortion of the formed product.

According to another preferred embodiment of the invention, amulti-layered container comprises a first polymeric layer, and secondintermediate or tie layer, and a third polymeric layer. Themulti-layered container is particularly useful for packaging foodstuffs(e.g., fresh meat, fish, or vegetables, prepared or semi-prepared foods,and the like) using gas flushed sealing or modified atmosphere packaging(MAP) with highly elastic films. The polyalkylene terephthalate ornaphthalate first layer provides stiffness and dimensional stability tothe container, thereby avoiding deformation due to stresses, for examplefrom the stretched lid stock. The first layer also provides thecontainer with excellent gas barrier properties. The third polymericlayer provides markedly improved adhesion with conventionalpolyethylene-based lid stock. The container, as a whole, exhibitssignificantly improved gas barrier properties and shelf life forpackaged foodstuffs.

The first layer, which usually forms the outside of the container,comprises an alkylene terephthalate or naphthalate polyester, such aspolyethylene terephthalate (PET), as previously defined for the bulkpolymer. The first layer may include, in whole or in part, virginpolymers, reprocessed or recycled polymers, post-consumer waste, andcombinations thereof, any of which can be homo-polymers or co-polymers.

Polyesters having lower I.V. generally have lower molecular weights,shorter chain lengths, and exhibit faster crystallization kinetics,resulting in better heat setting properties (e.g., higher dimensionalstability). In addition, lower-I.V. polymers generally are lessexpensive, and have lower extrusion melt temperatures, resulting in lessdegradation, faster stress relaxation time, reduced molding time andreduced production time. Given these properties, lower I.V. polyestersoften can be drawn into deeper molds even without the use of a plugassist. A preferred polyester is polyethylene terephthalate (PET) havingan I.V. of less than 0.95, 0.90, 0.85, or 0.80 (available from ShellPolyester). The thickness of the first layer should be suitably selectedto provide the desired level of dimensional stability and adequatebarrier properties to the container. Most often, the average thicknessranges from about 5 to 35 mils, more usually from about 10 to about 20mils, and even more usually from about 12 to about 18 mils.

The intermediate or tie layer primarily functions as an adhesive and asa compatibilizer/emulsifier/surfactant (CES) for the first and thirdlayers, which in a preferred embodiment are polar and non-polar,respectively. A preferred material for the intermediate or tie layer isa grafted or backbone-based co-polymer or ter-polymer comprisingethylene and a glycidyl acrylate, such as glycidyl methacrylate, and/ormaleic anhydride. The co-polymer or ter-polymer optionally includes oneor more other acrylate co-monomers such as methacrylate, ethylacrylate,propylacrylate, butylacrylate, ethylhexylacrylate, etc. Suitableexemplary amounts of glycidyl acrylate, maleic anhydride, or mixturethereof, range from about 0.05 wt % to about 12 wt %, typically fromabout 0.1 wt % to about 10 wt %, and more typically from about 0.8 wt %to about 9 wt %, based on the total weight of the co-polymer orter-polymer. A grafted co-polymer or ter-polymer typically will haveless glycidyl acrylate or maleic anhydride (e.g., from about 0.2 wt % toabout 1.5 wt %), whereas a backbone-based co-polymer or ter-polymer canhave higher amounts of glycidyl acrylate and/or maleic anhydride, e.g.,as indicated above. Suitable exemplary amounts of acrylate range from 0to about 40 wt %, preferably from about 10 wt % to about 30 wt %, andeven more preferably from about 20 wt % to about 35 wt %, based on thetotal weight of the co-polymer or ter-polymer. The melt flow index ofthe co-polymer or ter-polymer preferably is less than about 20, morepreferably is less than about 10, and even more preferably is less thanabout 6 g/10 min.

In one preferred embodiment, the intermediate or tie layer comprises ater-polymer of ethylene with about 8 wt % glycidyl methacrylate andabout 25 wt % methacrylate or butylacrylate, based on the total weightof the ter-polymer. In another preferred embodiment, the intermediate ortie layer comprises a co-polymer of ethylene with about 6 wt % glycidylmethacrylate, based on the total weight of the co-polymer. In anotherpreferred embodiment, the intermediate or tie layer comprises ater-polymer of ethylene with about 2 wt % glycidyl methacrylate and fromabout 17 to 25 wt % methacrylate, based on the total weight of theter-polymer. In yet another preferred embodiment, the intermediate ortie layer comprises a ter-polymer of ethylene with about 3 wt % maleicanhydride and about 17 wt % butylacrylate, based on the total weight ofthe ter-polymer. Where the first layer includes both reprocessed andvirgin polyesters, it may be advantageous to use a blend of the CESpolymer families (e.g., glycidyl methacrylate- and maleicanhydride-based) to alter the mode of failure so as to improve adhesion.Under failure conditions, the tie layer generally will delaminate fromthe polyethylene layer when using the glycidyl methacrylate-based CES,and from the polyester layer when using the maleic anhydride-based CES.

The second layer optionally comprises a blend of the CES co-polymer orter-polymer, as described above, and a polar co-polymer of ethylene anda C₁-C₁₂ acrylate, such as methacrylate, ethylacrylate, propylacrylate,butylacrylate, ethylhexylacrylate, or a mixture thereof. The polarco-polymer can be added, for example, to adjust rheology and/or toimprove adhesion, thermal stability, compatibility, and the like. Theamount of polar co-polymer used will depend on such factors as theco-monomer concentration in the CES. It has been found that improvedadhesion and compatibility actually can result when a polar co-polymeris blended with a high co-monomer CES (e.g., 12 wt % of glycidylmethacrylate) at a weight ratio up to about 4:1 (polar co-polymer toCES).

The second layer typically has an average thickness of at least about0.1 mils. There is no particular upper limit on the thickness; thepractical limiting factor is cost. Most often, the average thicknessranges from about 0.1 to about 2 mils, and more usually from about 0.2to about 1.5 mils.

The third layer comprises non-polar polyethylene. A preferred materialfor the third layer is high density polyethylene (HDPE) sold under thetrade name Chevron 9608, which has a melt flow index of 8 and a densityof 0.962. Alternatively, the third layer can comprise low densitypolyethylene (LDPE), linear low density polyethylene (LLDPE), or anycombination of HDPE, LDPE, and LLDPE. Because LDPE generally exhibitspoorer barrier properties than HDPE, it is preferred that no more thanabout 50 wt % LDPE is used, so as to avoid the need for an excessivelythick layer. An example of a highly branched LDPE that can be used isChevron grade 4517, which has a density of 0.923 and a melt flow indexof 5. Highly branched polymers typically exhibit improved seal strengthover time, but suffer from poorer functional barrier properties. Onepreferred linear low density polyethylene is Linear Low Density Chevron7325, which has a density of 0.925 and a melt flow index of 3.5. A blendof HDPE and LLDPE materials often exhibits improved compatibility withreprocessed polyesters and improved heat-setting properties. An exampleof a blend exhibiting improved hot tack sealing has a major portion ofHDPE and from about 10-20 wt % LLDPE.

Any or all of the polyethylene materials in the third layer can include,in whole or in part, metallocene-based polyethylene. Metallocene-basedpolyethylene materials generally have lower melting points and are moreamorphous, which improves heat-sealing properties.

The third layer typically will form the inside of a container and may beselected to provide aesthetics, color, gas barrier properties, and thelike. The third layer preferably is selected to improve sealing(adhesion) properties with lidding materials to be later applied ontothe container. Commonly used lid stocks include Cryovac 1050, Freshwrapand similar types sold by Cryovac and Packaging Partners. The averagethickness of the third layer in the final article most often is greaterthan about 1 mil, typically ranges from about 1 to about 5 mils, moretypically from about 2 to about 4 mils, and even more typically fromabout 3 to about 4 mils.

Foodstuffs can be packaged in the container using gas flushed sealing ormodified atmosphere packaging (MAP). In a typical operation, oxygen(O₂), nitrogen (N₂), and optionally carbon dioxide (CO₂) arecontrollably flushed into a container containing a foodstuff as thehighly elastic lid stock is stretched and sealed to the flange portionof the container. One of the reasons for using highly elastic materialsis to avoid sagging of the lid stock due to pressure changes inside thecontainer over time, e.g., as the foodstuff absorbs carbon dioxide.

An example of a three-layered thermoplastic container comprises a firstlayer comprising a polyethylene terephthalate co-polymer; a second layercomprising a grafted ter-polymer of ethylene and 30 wt % methacrylateand 0.8-1.5 wt % glycidyl methacrylate or maleic anhydride, based on thetotal weight of the ter-polymer; and a third layer comprising a blend ofHDPE and 10-20% LLDPE or Bynel E361 or 3060.

In an alternative embodiment of the present invention, the intermediateor tie layer comprises any suitable adhesive useful in adhering adjacentlayers of co-extruded films. In this embodiment, a minor amount of theCES is physically blended with the polyester together with recycled orreprocessed polymers in the first layer, typically in an amount of fromabout 1 to about 5 wt %. Examples of materials that can be used for theintermediate or tie layer in this embodiment include chemically modifiedethylene polymers, e.g., co-polymers of ethylene with esters ofethylenically unsaturated carboxylic acids, such as alkyl acrylates ormethacrylates, graft co-polymers of maleic acid or anhydride ontoethylene vinyl acetate copolymers, graft co-polymers of fused ringcarboxylic anhydrides onto polyethylene, resin mixtures of these, andmixtures with polyethylene or co-polymers of ethylene and alpha olefin.Such materials include adhesives sold under the tradename Bynel (duPont)or Admer (Mitsui).

The area stretch ratio of the multi-layered container most often rangesfrom about 1.25:1 to about 3:1, more usually from about 1.5:1 to about3:1. In one preferred embodiment, the area stretch ratio is about 2:1.

The thermoplastic materials, and in particular the alkyleneterephthalate or naphthalate polymer, can subjected to thermaltreatments in accordance with the intended use of the container.Preferably, the materials are heat set to provide dimensional stability,impact resistance, temperature resistance, and resistance to microwaveradiation. A level of crystallinity above 15% is preferable for adequatedimensional stability during demolding operations. A level above about20% is preferable to yield parts with excellent dimensional stabilityand impact resistance.

In an alternative multi-layered container embodiment, the thermoplasticmaterial is subjected to a thermal treatment that is specificallydesigned to cause the container to become visibly distorted when thecontainer contains foodstuffs and is exposed to microwave radiation.This can be done, for example, by molding the polyester at a lowertemperature and/or by maintaining the polyester at the moldingtemperature for a shorter time so as to achieve no more than minimalheat setting. The distortion, for example, can alert consumers that thecontainer is not intended for use in a microwave oven. Notwithstandingthe distortion, the container remains microwave-safe, i.e., does notcontaminate the food upon exposure to microwave radiation.

Alternatively, the containers can be heat set, as described, so as toprovide containers specifically designed for cooking applications suchas thawing frozen foods in a microwave oven, e.g., without distortion.The thermoplastic materials can be subjected to thermal treatments torender the container heat resistant and suitable for various othercooking applications, as desired.

Given the dissimilarities in the properties of the three layers (e.g.,melting point), three extruders preferably are used for processing thefirst, second, and third layers, respectively. It is preferred that theco-extrudate not be stretched or oriented in either direction. In atypical continuous process, the co-extrudate, after leaving theextrusion die and optionally being fed over cooling or shaping rollers,is contacted with a rotating wheel having a plurality of mold members aspreviously discussed. The time that the co-extrudate remains in contactwith the mold surface at the molding temperature can vary over a widerange and depends on a number of factors, such as the moldingtemperature, the dimensions of the mold, the number of molds, and thelike. Preferably, the co-extrudate remains in the mold for a timesufficient to heat-set the article. The time at the molding temperaturemost often ranges from about 10-30 seconds, more usually from about10-20 seconds.

A significant advantage of a continuous process over a discontinuousprocess is that the individual layers can be extruded and molded atdifferent temperatures in accordance with their individual thermoformingproperties, such as softening and melting points. The first layer(polyester) typically is extruded at a temperature of from about 450 to530° F. (about 232 to 276° C.). The intermediate or tie layer most oftenis extruded at a higher temperature to improve its adhesion properties,typically from about 480 to 550° F. (about 249 to 288° C.). The third,polyethylene layer typically is extruded at a temperature of from about425 to about 550° F. (about 218 to 288° C.).

By extruding each of the layers at their individual most suitablethermoforming temperatures, surface contact between the layers isdramatically improved and the likelihood of delamination is reduced oravoided. In addition, the vacuum in the mold often is sufficient toobtain adequate contact between the layers for adhesion, without theneed for a pressure box. It is desirable to avoid the use of a pressurebox not only because of increased process time and expense, but alsobecause of the problem of unwanted adhesion between the pressure box andthe upper (e.g., polyethylene) layer.

Preferably, the layers are co-extruded with the (thicker) first layer ontop, and a reversing roll is used to invert the co-extrudate prior tobeing contacted with the molding surface. When female molds are used,the first layer typically contacts the mold surface.

EXAMPLE 1

This example illustrates the improved retained I.V. and improvedtoughness characteristics of compositions of the present invention.

Three compositions (A, B, C) were used to prepare heat-set trays inaccordance with the continuous process described above. Each of thethree compositions included a bulk polymer that included 70 wt % ofShell 0.85 I.V. homopolymer, a toughener additive, and a glycidylmethacrylate-based CES. The remaining 30 wt % of the bulk polymers ofthe compositions was as follows. For composition “A”: Dupont 0.85+0.82I.V. homopolymer;

for composition “B”: KOSA 0.60−0.62 I.V. homopolymer; and forcomposition “C”: Reliance 0.80 I.V. co-polymer. Each of these polymersis commercially available. Table I summaries the final intrinsicviscosity of the articles, both uncorrected and corrected for 15%additives. TABLE I Final Intrinsic Viscosity Composition A Composition BComposition C I.V. uncorrected 0.661 0.605 0.629 I.V. corrected for0.765 0.706 0.736 15% additives

As can be seen from Table I, the compositions of the present inventionexhibit good retained I.V. The “I.V. corrected for 15% additives” isillustrative of the compositions' good retained I.V., e.g., bycomparison to the initial I.V. of the bulk polymers, which does notinclude additives. The initial viscosity of the compositions, as definedherein, would be less than the initial I.V. of the respective bulkpolymers because of the presence of the additives.

Containers made from compositions A, B, and C were subjected to impacttesting using standard Dynatup Impact equipment. Table II illustratesthe toughness results at 70° F. (21° C.), 32° F. (0° C.), and −20° F.(−29° C.). TABLE II Dynatup Impact Toughness Test Sample A Sample BSample C Dynatup Impact lbs. 137 167 134 @ 70° F. (21° C.) s.d. 5.0 s.d.6.1  s.d. 5.0  Dynatup Impact lbs. 158 200 154 @ 32° F. (0° C.) s.d. 7.6s.d. 9.2  s.d. 4.5  Dynatup Impact lbs. 173 211 173 @ −20° F. (−29° C.)s.d. 9.7 s.d. 35.2 s.d. 19.6 Thickness (mils)  18  25  20

As can been seen from Table II, surprisingly and inexplicably, thecontainers of the present invention were found to actually exhibitincreased toughness at lower temperatures.

EXAMPLE II

This example also illustrates the improved toughness properties of thecompositions of the present invention.

Trays were prepared using a 0.85 I.V. Shell 8506 homopolymer, atoughener additive, and a glycidyl methacrylate-based CES. One tray(“W”) was prepared using the continuous process described above, andthree trays (“X”, “Y”, and “Z”) were prepared using a discontinuousprocess, similar to that described in Gartland U.S. Pat. No. 4,469,270.The trays were tested for toughness using a standard Gardner Impacttest. FIG. 7 illustrates the mean failure energy at 73° F. (23° C.),−20° F. (−29° C.), and −31° F. (−35° C.). For each container, 20 sampleswere tested at 0.73° F. (23° C.) and at −20° F. (−29° C.), and 40samples were tested at −31° F. (−35° C.).

While particular embodiments of the present invention have beendescribed and illustrated, it should be understood that the invention isnot limited thereto since modifications may be made by persons skilledin the art. The present application contemplates any and allmodifications that fall within the spirit and scope of the underlyinginvention disclosed and claimed herein.

1-23. (canceled)
 24. A multi-layered thermoformed food tray comprisingfirst, second, and third layers heat-set into a rigid, dimensionallystable article having a bottom portion and a flange portion, wherein thefirst, second, and third layers comprise: a first polymeric layercomprising an alkylene terephthalate or naphthalate polymer, the firstpolymeric layer containing up to about 30 wt % mineral filler; a secondintermediate layer comprising a glycidyl or maleic functional co-polymerof ethylene or ter-polymer of ethylene and an acrylate, or both; and athird polymeric layer comprising a substantially non-polarthermoplastic.
 25. The food tray of claim 24 wherein the first layercomprises a blend of (i) virgin polymer and (ii) reprocessed polymericmaterials from the first, second, and third layers.
 26. The food tray ofclaim 24 wherein said first polymeric layer is selected from the groupconsisting of PET, PEN, PETG, PCT, PCTA, PBT, PTT, and mixtures thereof,and comprises one or more linear or branched homo-polymers, co-polymers,reprocessed polymers, recycled polymers, or a mixture thereof.
 27. Thefood tray of claim 24 wherein said second intermediate layer is selectedfrom the group consisting of ethylene/glycidyl methacrylate co-polymer,ethylene/maleic anhydride co-polymer, ethylene/glycidylmethacrylate/methacrylate ter-polymer, ethylene/glycidylmethacrylate/ethylacrylate ter-polymer, ethylene/glycidylmethacrylate/butylacrylate ter-polymer, ethylene/glycidylmethacrylate/ethylhexyl acrylate ter-polymer, ethylene/maleicanhydride/methacrylate ter-polymer, ethylene/maleicanhydride/ethylacrylate ter-polymer, ethylene/maleicanhydride/butylacrylate ter-polymer, ethylene/maleicanhydride/ethylhexyl acrylate ter-polymer, and mixtures thereof.
 28. Thefood tray of claim 27 wherein said second intermediate layer comprises agrafted co-polymer or ter-polymer.
 29. The food tray of claim 27 whereinsaid second intermediate layer comprises a blend of (i) said co-polymeror ter-polymer and (ii) a co-polymer of ethylene and an acrylate. 30.The food tray of claim 27 wherein said second intermediate layercomprises a blend of (i) said co-polymer or ter-polymer and (ii)polyethylene or co-polymers of ethylene and an alpha olefin.
 31. Thefood tray of claim 24 wherein said third polymeric layer is asubstantially non-polar polyolefin.
 32. The food tray of claim 24wherein said first polymeric layer is dimensionally distorted when thefood tray is filled with food and is exposed to microwave radiation. 33.The food tray of claim 24 wherein said first polymeric layer is heat setand wherein said food tray is suitable for cooking in a microwave oven.34. The food tray of claim 24 wherein said food tray contains foodstuffand is sealed with an elastic polyolefin-based lidding stock usingmodified atmosphere packaging.
 35. A multi-layered thermoformedmicrowavable food tray comprising first, second, and third layersheat-set into a rigid article which is dimensionally stable at elevatedtemperatures encountered in cooking applications, the article having abottom portion and a flange portion, wherein the first, second, andthird layers comprise: a first polymeric layer comprising polyethyleneterephthalate, the first polymeric layer containing up to about 30 wt %mineral filler; a second intermediate layer selected from the groupconsisting of ethylene/glycidyl methacrylate co-polymer, ethylene/maleicanhydride co-polymer, ethylene/glycidyl methacrylate/methacrylateter-polymer, ethylene/glycidyl methacrylate/ethylacrylate ter-polymer,ethylene/glycidyl methacrylate/butylacrylate ter-polymer,ethylene/glycidyl methacrylate/ethylhexylacrylate ter-polymer,ethylene/maleic anhydride/methacrylate ter-polymer, ethylene/maleicanhydride/ethylacrylate ter-polymer, ethylene/maleicanhydride/butylacrylate ter-polymer, ethylene/maleicanhydride/ethylhexylacrylate ter-polymer, and mixtures thereof; and athird polymeric layer comprising a substantially non-polar polyolefin;wherein the first layer comprises a blend of (i) virgin polymer and (ii)reprocessed polymeric materials from the first, second, and thirdlayers.
 36. The multi-layered thermoformed microwavable food tray ofclaim 35 wherein: said first polymeric layer has an average thickness offrom about 5 to about 35 mils; wherein said second intermediate layerhas an average thickness of from about 0.1 to about 2 mils; wherein saidthird polymeric layer has an average thickness of from about 1 to about5 mils; and wherein said container has an area stretch ratio of fromabout 1.5:1 to about 3:1.